1. Field of the Invention
This invention relates to article detection systems such as are used to protect articles from theft. More particularly the invention provides improvements which enhance the sensitivity and reliability of such article detection systems.
2. Description of the Prior Art
French Pat. No. 763,681 to Pierre Arthur Picard discloses a magnetic type article detection system which is used to protect library books from theft. In the Picard system an electrical coil is provided at the doorway of the library and it is electrically energized to generate a varying magnetic fundamental frequency field, through which all books must pass as they are taken from the library. The books are each provided with a target made of an elongated strip of ferromagnetic material of high magnetic permeability; and as this strip passes through the varying magnetic field at the doorway the field causes the strip to generate other fields at various harmonic frequencies. A sensing means is also provided at the doorway to sense the presence of those harmonic frequencies which are characteristic of the target material.
Picard also discloses that if the target material is of elongated configuration, the induction, i.e. its magnetic effect, will be high. A number of U.S. patents subsequent to Picard, i.e. U.S. Pat. Nos. 3,665,449, 3,697,996, 3,747,086, 3,765,007, 3,790,945, 3,820,103 and 3,820,104, say the same thing.
The Picard patent also discloses the use of target antennas which extend in two planes to decrease orientation sensitivity. This idea of a multidimensional target or responder to decrease orientation sensitivity is also recognized in U.S. Pat. No. 3,697,996. The term "orientation sensitivity" is used herein to mean the variation in the strength of the response signal produced by a target or responder strip when it is turned or reoriented with respect to the doorway coil which generates the fundamental frequency field.
It is important that the responder strip have minimal orientation sensitivity because books or other protected articles, on which the responder strips are mounted, may be oriented in any of several planes as they pass through the fundamental frequency field. It is, of course, possible to provide different strips arranged in different planes on the protected articles, or to provide "L" or "T" shaped strips, as recognized in the prior art, in order to minimize orientation sensitivity. However, such arrangements are bulky and expensive and they are often impractical for incorporation into various articles.